This invention relates to a high-frequency heater using a magnetron to execute dielectric heating, such as a microwave oven, and in particular to the configuration of a step-up transformer for driving a magnetron by using a switching power supply, and more particularly to a transformer comprising an inverter for converting large power from a commercial power supply into high-frequency, high-voltage power for driving a magnetron.
Further, this invention relates to a magnetron drive step-up transformer unit of a high-frequency heater using a magnetron to execute dielectric heating, such as a microwave oven, and in particular to a transformer structure to enhance the stability of a heater current for heating a magnetron filament and prevention performance of a short circuit of PS.
Hitherto, as this kind of magnetron drive step-up transformer using a switching power supply, a step-up transformer comprising a primary winding 1, a secondary winding 2, and a heater winding 3 placed in parallel on the same axis as U-shaped magnetic substances 4 and 5 as shown in FIG. 22 has been generally used for the following reason.
For a magnetron drive power supply often handling large power, use of a zero-volt switching technique (ZVS technique) based on voltage resonance is mainstream to lighten the load of a power semiconductor; in the ZVS technique, a step-up transformer coupling coefficient needs to be set in the range of about 0.6 to 0.85 to provide a resonance voltage. Thus, it is difficult to use a transformer of concentric multilayer winding (the coupling coefficient is 0.9 or more because the coupling between windings is strong).
However, in the described configuration in the related art, an attempt to produce higher output of the magnetron falls easily into a situation in which the peak current flowing into the primary side of the step-up transformer is more increased and thus the magnetic substances used with the step-up transformer are easily saturated. To solve this problem, it is necessary to upsize the magnetic substance, namely, the transformer, causing a bottleneck in pursuing miniaturization of the power supply.
FIG. 23 is an external view of a step-up transformer in a related art. In the figure, numeral 201 denotes a bobbin made of a resin around which a primary winding 202, a secondary winding 203, a short proof winding 204 for preventing the primary and secondary windings from being shorted, and a heater winding 205 for supplying power for heating a cathode of a magnetron are wound. The portion of the bobbin 201 around which the secondary winding 203 is wound is divided into four winding grooves by partitions 206. First, the secondary winding is wound around the first winding groove and upon completion of a predetermined amount of the winding, then the remaining winding is wound around the second winding groove. Upon completion of a predetermined mount of the winding, then the remaining winding is wound around the third winding groove. Then, the remaining winding is wound around the fourth winding groove, so that the division winding of the secondary winding is completed. Thus, if aligned winding is not accomplished in each winding groove and partial winding disorder occurs, the division grooves around which the secondary winding is wound are insulated with resin and thus the risk of producing an electrical breakdown between the parts of the secondary winding can be excluded. Since the secondary winding consists of the winding divisions, heat generated by a copper loss on the secondary winding is dispersed into the winding grooves and is radiated, so that an excellent radiation characteristic, namely, an advantage of suppressing a temperature rise can be provided. Numeral 207 denotes a core made of ferrite, etc. The core 207 serves a magnetic circuit for transferring magnetic energy generated by an electric current of the primary winding 202 to the secondary winding 203. A resin core cover 208 for insulating the core 207 and the windings is attached. The description of the step-up transformer in the related art is now complete.
However, such a step-up transformer comprises the primary and secondary windings placed in parallel, thus a method of widening the winding width of the primary winding 202 for enlarging the exposure area for cooling or widening the winding width of the secondary winding 203 or increasing the number of divisions of the secondary winding 203 for enlarging the exposure area for providing a good heat radiation property is available to provide a good radiation property and temperature performance.
To use the step-up transformer with a microwave oven, etc., increasing output is an indispensable factor for speedy heating; to increase output, the energy transferred with the transformer also grows and a temperature rise needs to be suppressed for avoiding degradation of insulation performance. Thus, to provide a good heat radiation property for lowering the temperature, the transformer must be widened and upsized; this is a problem.
Further, hitherto, for this kind of inverter power unit, high-voltage power supply for driving a magnetron, the Unexamined Japanese Patent Application Publication No. Hei 5-121159 discloses a monolithic voltage resonance inverter of a single-terminal type. The inverter power unit converts power converted into a high frequency by the inverter into a high voltage through a step-up transformer and generates a high DC voltage appropriate for driving the magnetron by a voltage doubler rectifier circuit. According to the configuration, the step-up transformer can be miniaturized by converting power into a high frequency by the inverter and the circuitry is formed on a single board, so that a compact and lightweight magnetron drive power supply (inverter power supply) can be provided.
In the described configuration in the related art, an attempt to produce higher output of the magnetron falls easily into a situation in which the peak current flowing into the primary side of the step-up transformer is more increased and thus the magnetic substances used with the step-up transformer are easily saturated. To solve this problem, it is necessary to upsize the magnetic substance, namely, the transformer, causing a bottleneck in pursuing miniaturization of the power supply.
Still further, hitherto, for an inverter power unit for converting a commercial power supply into a high-frequency, high-voltage power supply for driving a magnetron, the Unexamined Japanese Patent Application Publication No. Hei 5-121159 discloses a monolithic voltage resonance inverter of a single-terminal type. The inverter power unit converts power converted into a high frequency by the inverter into a high voltage through a step-up transformer and generates a high DC voltage appropriate for driving the magnetron by a high-voltage circuit using multiplication voltage rectification or a rectifier circuit, whereby the step-up transformer can be miniaturized by converting power into a high frequency by the inverter and the circuitry is formed on a single board, so that a compact and lightweight magnetron drive power supply (inverter power supply) can be provided.
FIG. 24 is a side view of a step-up transformer 408 of a magnetron drive step-up transformer unit in a related art. In the figure, numeral 401 denotes a bobbin made of a resin around which a primary winding 402, a secondary winding 403, and a heater winding 404 for supplying power for heating a cathode of a magnetron are wound. Numeral 405 denotes a core made of a magnetic substance of a ferrite, etc. The core 407 serves the function of a magnetic circuit for transferring magnetic energy generated by an electric current of the primary winding 402 to the secondary winding 403. The windings are bound on terminal pins 406 and are dip-soldered. The step-up transformer 408 has the described configuration. The terminal pins 406 are inserted into holes made in a printed circuit board 407 and are dip-soldered.
On the other hand, FIG. 25 is an example of a circuit diagram of the magnetron drive step-up transformer unit, which is a typical monolithic voltage resonance type inverter, generally used with an electromagnetic cooker, a rice cooker, etc., including a microwave oven.
A full-wave rectifier circuit 410 converts a commercial power supply 409 into a unilateral power supply. A rectification filter 413 consisting of a choke coil 416 and a smoothing capacitor 417 smoothes an electric current and blocks leakage and entry of noise. Inductance viewed from the primary side of the step-up transformer 408 and a resonance capacitor 414 make up a resonance circuit. Numeral 415 denotes a switching element made up of an IGBT (insulated gate bipolar transistor) and an FWD (forward diode). The parts make up an inverter section 416 of a switching circuit; a DC power supply of the rectification filter 413 is fast switched by the switching element 415 and is converted into a high-frequency power supply. High-frequency power is converted into high-frequency high-voltage power by the step-up transformer 408 and is further converted into a high DC voltage by a half-wave voltage doubler circuit 420 made up of high-voltage diodes 417 and 418 and a high-voltage capacitor 419, driving a magnetron 421 for generating microwave energy.
On the other hand, power is supplied from the heater winding 404 to the filament of the magnetron 421 for raising temperature, thereby promoting excitation of electrons. A control circuit 422 controls the inverter section 416 and the amount of microwave energy generated from the magnetron 421 is controlled by controlling the switching element 415. To block leakage of high-frequency noise from the magnetron 421, the power supply line to the filament is provided with a choke coil 423. In such an inverter system similar to monolithic voltage resonance of single terminal type, generally the control circuit 422 changes the conduction time of the switching element 415 for controlling power supply to the magnetron 421.
According to the control technique, the energy given to foods can be changed as desired and means for controlling power for warming foods up linearly can be provided. This is a feature that cannot be provided by a conventional iron-type transformer; at the time, the operation frequency of the inverter changes. As the power is lowered, the conduction time of the magnetron current flowing is shortened, and the voltage of the secondary winding is lowered remarkably as compared with the primary winding. Therefore, if the heater winding is coupled with the primary winding more strongly than with the secondary winding as much as possible, the current flowing into the cathode of the magnetron 421 at the power control time is less changed. As in the transformer in FIG. 24, the heater winding 404 is provided between the primary winding 402 and the secondary winding 403. The time change of anode-cathode voltage ebm and filament current If of the magnetron at the time becomes as shown in FIG. 27, and If is comparatively less changed by power control.
If tends to lower with lower output because of power transfer attenuation caused by the inductance component of the choke coil 423 caused by higher frequency at the lower time and coupling of the secondary winding, but it is advantageous for moding and it is made possible to narrow high-frequency output to low power.
On the other hand, considering a safety standpoint, the heater winding 404 for generating the highest voltage in the secondary circuit is brought close to the primary winding 402 and the configuration is extremely dangerous from the viewpoint of a short circuit of PS. Particularly, the secondary side is a high voltage and thus in the circuitry of a microwave oven wherein the cabinet chassis is at ground potential of the secondary side, if the chassis is not grounded, when a short circuit of PS occurs, the cabinet floats at a high voltage of about 4 kV to 7 kV; the configuration is extremely dangerous for the user.
Placing emphasis on the safety standpoint, the configuration as shown in FIG. 26 is possible, wherein the dangerous heater winding is placed on the opposite side to the primary winding 402. In this case, the risk of a short circuit of PS can be almost circumvented and the configuration is safety.
However, seeing the time change of the filament current If in FIG. 27, the value of If tends to extremely lower in response to power control; this is extremely disadvantageous from the viewpoint of moding of the magnetron.
Thus, a trade-off occurs between safety and performance.
Still further, hitherto, in this kind of magnetron drive step-up transformer using a switching power supply, a primary winding 501, a secondary winding 502, and a heater winding 503 have been wound around one bobbin 504 and have been placed in parallel on the same axis as U-shaped magnetic substances 505 and 506, as shown in FIG. 28. A primary winding terminal part 507 has been placed in a bobbin flange part 508 adjacent to the primary winding 501 of the bobbin 504 and a secondary winding terminal part 509 and a heater winding terminal part 510 at high potential have been placed in a bobbin flange part 511 adjacent to the heater winding 503 of the bobbin 504 considering an insulation structure of the primary winding 501 and the primary winding terminal part 507, as shown in FIG. 29. Insertion holes 513 for inserting the primary winding terminal part 507, the secondary winding terminal part 509, and the heater winding terminal part 510 have been made in a printed circuit board 512 on which a high-voltage circuit, a magnetron heater, and a switching circuit are print-wired for fixing the step-up transformer; after the terminal parts are inserted, solder fixing and print wiring have been carried out.
The described magnetron drive step-up transformer in the related art has the advantage that more than one winding can be wound around one bobbin; however, it involves the following problem in a state in which the step-up transformer is soldered, fixed, and wired on the printed circuit board: The primary winding terminal part print-wired to the switching circuit connected to a commercial power supply and the secondary winding terminal part and the heater winding terminal part printed-wired to the high-voltage circuit and the magnetron heater at high potential are connected in margins of the back and the surface of the printed circuit board. If dust should be deposited on the printed circuit board or dew condensation should occur on the printed circuit board due to salt air in a seaside area, there is a possibility of a high-voltage short circuit from the secondary winding terminal part or the heater winding terminal part at high potential to the primary winding terminal part because of margin discharge.
To prevent the accident from occurring, as shown in FIG. 30, a copper foil part 515 print-wired at the same potential as chassis ground is printed at a position between the primary winding terminal part 507 and the secondary winding terminal part 509 and the heater winding terminal part 510 on the copper foil print side of the printed circuit board 514 and discharge from the secondary winding terminal part 509 or the heater winding terminal part 510 at high potential is guided into the copper foil part 515 and is connected to the chassis ground for preventing a high-voltage short circuit to the primary winding terminal part 507. A conductive metal part 516 is placed at a position between the primary winding terminal part 507 and the secondary winding terminal part 509 and the heater winding terminal part 510 on the parts mount side of the printed circuit board 514 and moreover the metal part 516 is print-wired so as to become the same potential as the chassis ground, whereby discharge from the secondary winding terminal part 509 or the heater winding terminal part 510 at high potential is guided into the metal part 516 and is connected to the chassis ground for preventing a high-voltage short circuit to the primary winding terminal part 507. If a conductive metal part cannot be placed because of the structure, a printed circuit board 517 is formed with a slit 518 to provide the margin distance between the primary winding terminal part 507 and the secondary winding terminal part 509 and the heater winding terminal part 510 as shown in FIG. 31, whereby margin discharge from the secondary winding terminal part 509 or the heater winding terminal part 510 at high potential to the primary winding terminal part 507 is made hard to occur in the presence of the slit 518.
However, to thus prevent a high-voltage short circuit from the secondary winding terminal part or the heater winding terminal part at high potential to the primary winding terminal part by executing margin discharge on the parts mount side of the printed circuit board, a metal part needs to be placed at a position between the primary winding terminal part and the secondary winding terminal part and the heater winding terminal part of the magnetron drive step-up transformer or the printed circuit board needs to be formed with a slit. To place a metal part, insulation between the metal part and the windings must also be provided and thus the magnetron drive step-up transformer is enlarged in the height direction; this is a problem. To form a slit, the distance across the bobbin flange for placing the primary winding terminal part, the secondary winding terminal part, and the heater winding terminal part, namely, the winding width direction of the bobbin of the magnetron drive step-up transformer must be enlarged, and since the printed circuit board is formed with a slit, the risk of breaking the printed circuit board due to drop or vibration is increased; this is also a problem.
In the invention, to solve the problem, a primary winding and a secondary winding are layer-wound concentrically with respect to the magnetic substance forming a main magnetic circuit and the magnetic substance forming a subordinate magnetic circuit is placed between the primary winding and the secondary winding, whereby a leakage flux is generated.
According to the invention, the subordinate magnetic circuit placed between the primary winding and the secondary winding makes it possible to control the leakage amount between the primary winding and the secondary winding, and any desired coupling coefficient can be provided in a concentric multilayer winding transformer.
The concentric multilayer winding transformer, which has strong coupling between windings, has a feature of making the magnetic substance hard to be saturated even for a large current; consequently, the step-up transformer can be miniaturized effectively even for higher output.
According to the invention as in aspect 1, 2, or 3, it is made possible to adjust the coupling coefficient as desired in the presence of the subordinate magnetic circuit, the concentric multilayer winding transformer can provide a magnetron drive step-up transformer adopting the ZVS technique, and a miniaturized power supply that can prevent a magnetic substance from being saturated with higher output can be realized.
According to the invention as in aspect 4, 5, or 6, the leakage flux amount can be adjusted for providing any desired coupling coefficient.
It is therefore an object of the invention to provide a transformer having a primary winding and a secondary winding concentrically with the primary winding placed outside and the secondary winding placed inside and having a predetermined space provided between the primary winding and the secondary winding.
According to the invention as in aspects 7, 8, or 9, it is made possible to place the secondary winding in the inside of the bobbin of the primary winding and the space factor can be lessened drastically. Moreover, the air layer is provided between the primary winding and the secondary winding for enhancing insulation of the primary winding and the secondary winding, so that an unsafe mode of a short circuit of PS, etc., is hard to occur, and the whole of the primary winding which a large current of several ten amperes flows into and generates large heat is exposed to the outside atmosphere, thus the cooling performance is improved remarkably and high output of a microwave oven can be produced using a compact step-up transformer.
In the invention, to solve the problem, a magnetron drive step-up transformer comprises a primary winding, a secondary winding being placed inside the primary winding with a predetermined gap therebetween, and a subordinate magnetic circuit for generating a leakage flux between the primary winding and the secondary winding, wherein the subordinate magnetic circuit comprises an E-shaped magnetic substance placed at one end outside the primary winding and the secondary winding and placed at an opposite end between the primary winding and the secondary winding.
According to the invention as in aspect 10, there is provided a magnetron drive step-up transformer comprising a primary winding, a secondary winding being placed inside the primary winding with a predetermined gap therebetween, and a subordinate magnetic circuit for generating a leakage flux between the primary winding and the secondary winding, wherein the subordinate magnetic circuit comprises an E-shaped magnetic substance placed at one end outside the primary winding and the secondary winding and placed at an opposite end between the primary winding and the secondary winding, whereby the space factor can be lessened drastically. Moreover, the air layer is provided between the primary winding and the secondary winding for enhancing insulation of the primary winding and the secondary winding, so that an unsafe mode of a short circuit of PS, etc. , is hard to occur, and the whole of the primary winding which a large current of several ten amperes flows into and generates large heat is exposed to the outside atmosphere, thus the cooling performance is improved remarkably and high output of a microwave oven can be produced using a compact step-up transformer.
The subordinate magnetic circuit placed between the primary winding and the secondary winding makes it possible to adjust the coupling coefficient as desired, the concentric multilayer winding transformer can provide a magnetron drive step-up transformer using a voltage resonance technique, and a miniaturized power supply that can prevent a magnetic substance from being saturated with higher output can be realized.
In the invention as in aspect 11, the thickness and gap of the E-shaped magnetic substance are determined so that the coupling coefficient is set to 0.7 to 0.9 by adjusting the magnetism of the subordinate magnetic circuit. Thus, if the thickness of the E-shaped magnetic substance is examined, a stable area with magnetism unsaturated can be set and if the gap of the E-shaped magnetic substance is adjusted, the coupling coefficient can be adjusted to 0.7 to 0.9. Therefore, a hole of the optimum dimensions of an outer bobbin responsive to the thickness of the E-shaped magnetic substance is determined and a small and compact step-up transformer can be provided.
In the invention as in aspect 12, a spacer placed in a gap of the subordinate magnetic circuit is molded integrally with an outer bobbin, whereby the difficulty of attaching a small spacer to the depth of a hole for inserting the opposite end of the E-shaped magnetic substance is excluded, and the E-shaped magnetic substances are inserted from both sides of the hole, whereby the opposite ends of the E-shaped magnetic substances do not come in direct contact each other and the resin spacer is placed between the opposite ends of the E-shaped magnetic substances; an abnormal sound offensive to ears is not produced and the spacer need not again be inserted, namely, workability is good.
In the invention, to solve the problem, a step-up transformer comprises three types of windings of a primary winding, a secondary winding, and a heater winding, magnetic substances for transferring power of a switching circuit as a magnetic flux from the primary winding to another winding, an outer bobbin around which the primary winding is wound, and an inner bobbin around which the secondary winding and the heater winding are around, the inner bobbin being placed inside the outer bobbin, the windings being wound as concentric layers with respect to a magnetic circuit of the magnetic substance.
According to the invention, in the outer bobbin comprising the primary winding, the inner bobbin comprising the secondary winding and the heater winding is inserted as a two-piece structure, so that a short circuit of PS is extremely hard to occur in the safe structure and the heater winding exists just below the primary winding, thus the magnetic coupling between the primary winding and the heater winding is high, magnetic flux change at the power control time is small, and change in the current flowing into a filament is also lessened. Therefore, a magnetron drive step-up transformer unit having stable performance wherein change in filament current If is reduced and moding is hard to occur to low output can be provided.
According to the invention as in aspect 13, there is provided a magnetron drive step-up transformer unit comprising a magnetron, a step-up transformer for supplying a drive voltage to the magnetron, and a switching circuit being connected to the primary side of the step-up transformer, wherein the step-up transformer comprises three types of windings of a primary winding, a secondary winding, and a heater winding, magnetic substances for transferring power of the switching circuit as a magnetic flux from the primary winding to another winding, an outer bobbin around which the primary winding is wound, and an inner bobbin around which the secondary winding and the heater winding are around, the inner bobbin being placed inside the outer bobbin, the windings being wound as concentric layers with respect to a magnetic circuit of the magnetic substance.
Thus, the insulation of the primary and secondary windings is enhanced, variation in the filament current at the power change time is lessened, and moding becomes hard to occur.
In the invention as in aspect 14, the windings are wound as concentric layers with respect to the magnetic substance forming a main magnetic circuit and the outer bobbin and the magnetic substance forming a subordinate magnetic circuit are between the primary winding and the secondary winding and the heater winding.
Thus, the subordinate magnetic circuit makes it possible to adjust the coupling coefficient as desired, excitation of resonance voltage is intensified, stable zero-voltage switching can be realized, and the switching loss of the switching element can be decreased remarkably.
In the invention, to solve the problem, a first winding, a second winding, and a third winding of a step-up transformer are wound as concentric layers around two or more bobbins with respect to a magnetic substance forming a magnetic circuit, the bobbins are provided at appropriate positions with terminal parts of the windings, one of the bobbins is provided with a terminal part of magnetic substance ground brought into contact with the magnetic substance, the terminal part of the first winding and the terminal parts of the second and third windings are placed so as to face each other with the magnetic substance between, the winding terminal parts of the step-up transformer are soldered and fixed to a printed circuit board, and the first winding is connected to a switching circuit, the second and third windings are connected to a high-voltage circuit and a heater of a magnetron, and the terminal part of the magnetic substance ground is connected to chassis ground.
According to the invention, if dust should be deposited on the printed circuit board or dew condensation should occur on the printed circuit board due to salt air in a seaside area and a state should be entered in which margin discharge easily occurs in the direction of the terminal part of the first winding connected to the switching circuit from the terminal parts of the second and third windings connected to the high-voltage circuit and the heater of the magnetron at high potential, the terminal parts of the second and third windings face the terminal part of the first winding with the magnetic substance between, so that discharge occurs from the terminal parts of the second and third windings to the magnetic substance and an electric current flows into the chassis ground connected through the magnetic substance ground terminal from the magnetic substance, making it possible to prevent a high-voltage short circuit to the terminal part of the first winding connected to the switching circuit.
Therefore, the configuration eliminates the need for placing a conductive metal part at a position between the terminal part of the first winding and the terminal parts of the second and third windings on the printed circuit board and moreover print-wiring the metal part so as to become the same potential as the chassis ground as in the related art and also eliminates the need for enlarging the margin distance by providing a slit at a position between the terminal part of the first winding and the terminal parts of the second and third windings on the printed circuit board; the step-up transformer or the printed circuit board can be miniaturized and the strength can also be increased against breakage of the printed circuit board due to drop or vibration.
In other words, the step-up transformer can be miniaturized although output is made large, and the power supply can be miniaturized.
According to the invention as in aspect 15, there is provided a magnetron drive step-up transformer unit comprising a magnetron, a high-voltage circuit for supplying a high voltage to the magnetron, a step-up transformer for supplying a drive voltage to a heater of the magnetron and the high-voltage circuit, a switching circuit being connected to the primary side of the step-up transformer, and a printed circuit board on which the high-voltage circuit, the heater of the magnetron, and the switching circuit are print-wired for fixing the step-up transformer wherein a first winding, a second winding, and a third winding of the step-up transformer are wound as concentric layers around two or more bobbins with respect to a magnetic substance forming a magnetic circuit, wherein the bobbins are provided at appropriate positions, for example, flange part with terminal parts of the windings, wherein one of the bobbins is provided with a terminal part of magnetic substance ground brought into contact with the magnetic substance, wherein the terminal part of the first winding and the terminal parts of the second and third windings are placed so as to face each other with the magnetic substance between, wherein the winding terminal parts of the step-up transformer are soldered and fixed to the printed circuit board, and wherein the first winding is connected to the switching circuit, the second and third windings are connected to the high-voltage circuit and the heater of the magnetron, and the terminal part of the magnetic substance ground is connected to chassis ground.
Therefore, if dust should be deposited on the printed circuit board or dew condensation should occur on the printed circuit board due to salt air in a seaside area and a state should be entered in which margin discharge easily occurs in the direction of the terminal part of the first winding connected to the switching circuit from the terminal parts of the second and third windings connected to the high-voltage circuit and the heater of the magnetron at high potential, the terminal parts of the second and third windings face the terminal part of the first winding with the magnetic substance between, so that discharge occurs from the terminal parts of the second and third windings to the magnetic substance and an electric current flows into the chassis ground connected through the magnetic substance ground terminal from the magnetic substance; a high-voltage short circuit to the terminal part of the first winding connected to the switching circuit can be prevented.
In the magnetron drive step-up transformer unit as in aspect 16, the space distance between the terminal part of the second winding and the magnetic substance is made smaller than a half the space distance between the terminal part of the first winding and the terminal part of the second winding and the space distance between the terminal part of the third winding and the magnetic substance is made smaller than a half the space distance between the terminal part of the first winding and the terminal part of the third winding. The effect of preventing a high-voltage short circuit can be made larger.